1. Field of the Invention
The present invention relates to a superwide-angle lens system that is suitable for use in a mirrorless exchangeable-lens camera in which a split-mirror (quick-return mirror) for an optical finder is omitted, a compact camera, a mobile phone camera, surveillance camera and an in-vehicle camera.
2. Description of Related Art
In a superwide-angle lens system for a single-lens reflex (SLR) camera, in order to avoid interference from an optical finder split-mirror that is positioned in between the lens system and the imaging plane, it is necessary to attain a long backfocus relative to the focal length (i.e., the conditions for the backfocus are strict).
Whereas, in a superwide-angle lens system for a mirrorless exchangeable-lens camera, since a split-mirror for an optical finder is omitted, there is no need to attain a long backfocus relative to the focal length (i.e., the conditions for the backfocus are more relaxed). In such a type of superwide-angle lens system, if the number of lens elements thereof is increased with the aim to attain a higher optical quality, the size of the entire lens system is enlarged and the overall cost thereof increases. Whereas, if the number of lens elements is reduced (down to, e.g., four lens elements) in order to further miniaturize and reduce the overall cost thereof, the optical quality of the lens system deteriorates. Furthermore, if a wide attachment is used, the lens system is enlarged and the imaging quality deteriorates, so that the optical quality deteriorates.
A superwide-angle lens system that does not require a long backfocus relative to the focal length (the conditions for the backfocus are relaxed) is known in the related art, and is configured of a negative first lens group and a positive second lens group, wherein the negative first lens group is configured of a negative lens element, a negative lens element and a positive lens element (i.e., three lens elements), in that order from the object side, and the second lens group is configured of a negative lens element, a positive lens element, and a positive lens element (i.e., three lens elements), in that order from the object side (Japanese Unexamined Patent Publication Nos. H04-68307, 2003-232998, and 2005-221920).
However, in the superwide-angle lens system taught in Japanese Unexamined Patent Publication No. H04-68307, the refractive power of each of the two negative lens elements provided in the negative first lens group is small, so that it is difficult to widen the angle-of-view. Furthermore, due to the balance between the positional shift (peak shift), between the optimum imaging plane with respect to a high-frequency object and the optimum imaging plane with respect to a low-frequency object caused by negative spherical aberration, and the field curvature, the above-mentioned superwide-angle lens system taught in Japanese Unexamined Patent Publication No. H04-68307 does not have a sufficient optical quality for an image sensor having a large number of pixels.
In the superwide-angle lens systems disclosed in Japanese Unexamined Patent Publication Nos. 2003-232998 and 2005-221920, since about two or three plastic lens elements are used, a high design performance is attained using aspherical surfaces (formed on the plastic lens elements), and the overall cost is reduced due to the utilization of inexpensive plastic lens elements. However, a large amount of focal shift occurs in plastic lens elements upon a change in temperature, thus causing problems with variations in optical quality and lacking sufficient durability.